Hello, Night-Listeners.
I had an experience earlier today that made me realize that one of my
long-held assumptions about "dawn flights" hasn't been right.
In Lafayette, Boulder County, Colorado, there was a very heavy (by our
Western standards) dawn flight of warblers and sparrows during the half
hour of civil twilight this morning, Monday, August 25th--easily 500+
flight calls heard, for a rate of 1,000+ per hour. Again, that's a very
good flight for Colorado.
Now, my assumption would have been that this dawn flight would have been
preceded by a good night flight.
But that's not what happened. During the preceding period of
astronomical twilight, I heard only a single flight call--that of a
Wilson's Warbler. (An aside: Wilson's Warbler is far and away the most
abundant migrant warbler--often outnumbering all other warblers
combined--in the Front Range region in late August and early September.)
Here's what I think happened. During astronomical twilight, there were
moderate southerly winds aloft, evidenced by the uniform movement of the
low cloud ceiling relative to the barely visible glow of the moon.
That's no good if you're a southbound migrant. But there was absolutely
no wind at the surface at least up to the treetops. That's nice, but
with the cloud cover, there was no light during astronomical
twilight--so it wouldn't have been good for visually oriented birds
moving at or below the level of the treetops.
During civil twilight, however, the birds could see, so they got up and
went. It was fascinating to actually see them--MacGillivray's Warblers
and juvenile Chipping Sparrows, for example--in sustained flight 10-25
feet above street level in my residential neighborhood. Judging by all
the calling, I would say the flight continued until about 90 minutes
after sunrise. The flight was extremely low to the ground; I think these
birds were circumventing the southerly winds aloft by "flying under the
radar," so to speak, where winds were calm, and thus favorable for
migration.
I wouldn't say this dawn flight was a case of "onward migration,"
because that implies the birds had been migrating by night. I don't
think that happened. I think the southerly winds aloft prevented it. But
once they could see, the got up and migrated in the calm conditions near
the surface.
Or maybe I'm full of crap. In any event, it was cool to witness a very
heavy dawn flight but without any "precursor" in the form a nocturnal
flight.
---
Ted Floyd
Editor, Birding
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